1. Field of Invention
The present invention relates generally to semiconductor processing equipment. More particularly, the present invention relates to a mechanism which enables an actuator push point to be efficiently adjusted to minimize pitching moments associated with a stage apparatus.
2. Description of the Related Art
For precision instruments such as photolithography machines which are used in semiconductor processing, factors which affect the performance, e.g., accuracy, of the precision instrument generally must be dealt with and, insofar as possible, eliminated. When the performance of a precision instrument is adversely affected, as for example by pitching moments, products formed using the precision instrument may be improperly formed and, hence, function improperly. For instance, a photolithography machine which is subjected to pitching moments may cause an image projected by the photolithography machine to move, and, as a result, be aligned incorrectly on a projection surface such as a semiconductor wafer surface.
Scanning stages such as wafer scanning stages and reticle scanning stages are often used in semiconductor fabrication processes, and may be included in various photolithography and exposure apparatuses. Wafer scanning stages are generally used to position a semiconductor wafer such that portions of the wafer may be exposed as appropriate for masking or etching. Reticle scanning stages are generally used to accurately position a reticle or reticles for exposure over the semiconductor wafer. Patterns are generally resident on a reticle, which effectively serves as a mask or a negative for a wafer. When a reticle is positioned over a wafer as desired, a beam of light or a relatively broad beam of electrons may be collimated through a reduction lens, and provided to the reticle on which a thin metal pattern is placed. Portions of a light beam, for example, may be absorbed by the reticle while other portions pass through the reticle and are focused onto the wafer.
To reduce pitching moments which arise within a photolithography or exposure apparatus and, hence, to increase the accuracy with which a photolithography process or an exposure process may occur, the apparatus may be designed such that an actuator which drives a stage of the apparatus has a push point of center of force which is approximately in the same plane as the center-of-gravity of the stage. FIG. 1a is a diagrammatic representation of an overall stage apparatus. A stage apparatus 100 includes a wafer stage 104 which is arranged to perform coarse scans and a wafer table 108 which is arranged to perform fine scans. A wafer (not shown) is arranged to be held by a wafer chuck 110 which is generally supported on wafer table 108. Actuators 112 enable wafer table 108 to scan.
A magnet 116 which is generally positioned within wafer stage 104 is arranged to cooperate with a guide bar 120 to allow wafer stage 104 to scan over guide bar 120. Guide bar 120 is coupled on each end to a linear motor coil 124 which, together with a stator 128, serve as a motor which enables wafer stage 104 to scan along a z-axis 130.
When wafer stage 104 is driven using motors which include stators 128 (having a magnet or magnets) and motor coils 124, wafer stage 104 is expected to be driven through a center-of-gravity 140 of wafer stage 104 in order to substantially minimize the occurrence of pitching moments within stage apparatus 100. As such, to better enable wafer stage 104 to be driven through center-of-gravity 140, the location of the motors which include stators 128 and motor coil 124 is desired to be such that the push points associated with the motors are effectively aligned with center-of-gravity 140. When the push points associated with the motors are not in the same plane as center-of-gravity 140, e.g., the push points are below center-of-gravity with respect to a y-axis 132, then pitching moments which cause disturbances may arise.
A push point associated with a motor coil, as for example motor coil 124a, is typically located at a center of motor coil 124a. FIG. 1b is a diagrammatic representation of stator 128a and a portion of motor coil 124a of FIG. 1a. A push point 150 is effectively a center of force associated with coil 124a. Push point 150 is positioned such that force is generated on the center of coil 124a. In general, motor coil 124a may take on substantially any geometry. By way of example, motor coil 124a and stator 128a may be part of a three-phase motor such as a three-phase commutated sinusoidal motor. Connections 160 to coil 124a in a three-phase motor may be associated with a bridge, e.g., bridge 170 as shown in FIG. 1c. 
In many cases, aligning motors such that push points or centers of force of the motors closely correspond to a center-of-gravity of a structure that is being driven by the motors is difficult. For instance, identifying an exact location of a center of gravity of the structure is typically difficult. As a result, a push point of an actuator and the center of gravity of a structure, e.g., a stage, that is to be driven by the actuator may be out of alignment. Often, the error in alignment between the push point of an actuator and the center of gravity of a stage may be in the range of between approximately five mm and approximately ten mm. In order to compensate for errors in alignment, masses may be added to the stage in order to effectively move the center-of-gravity of the stage to more closely correspond to the push point of the actuator. Since the masses which generally must be added to a stage in order to move the center-of-gravity of the stage by between approximately five mm and approximately ten mm are relatively large, i.e., heavy, such masses may make it more difficult for the actuator to move the stage.
Therefore, what is needed is a method and an apparatus which enables the push point of an actuator to be readily aligned with a center-of-gravity of a stage which is being driven by the actuator. That is, what is desired is a system for enabling the push point of an actuator to be shifted to coincide with a center-of-gravity of a stage substantially without requiring that masses be added to the stage to shift the center-of-gravity.